Mount St. Helens

Credit:

NASA images by Robert Simmon, based on Landsat data. Caption by Lisa Anjozian.

Most of the geologic processes that shape our planet, such as the creeping movement of tectonic plates, are often too slow to see on human timescales. But every so often, geology produces a moment with in-your-face intensity.

The explosive eruption of Mount St. Helens on May 18, 1980 was such a moment. Situated on a subduction zone where the Juan de Fuca Plate in the Pacific Ocean descends under the North American Plate, the Cascade Range from British Columbia to northern California is dotted with volcanoes. Mount St. Helens is the most active volcano of the group, sitting over an area where the Earth’s surface holds in the melted solid rock and pressure builds up.

Landsats 2, 3, 5, and 7 have captured views of the Mount St. Helens eruption and the recovery of the ecosystem over the past thirty-two years. In the false-color image above from September 24, 1980, vegetation is red. See the rest of the time series in our World of Change section.

Before the eruption, Mt. St. Helens had a conical, glacier-clad peak similar to the others in the Cascades chain, and the volcano had been inactive since the mid-19th century. Scientists began actively monitoring the area in March 1980 when the volcano “reawakened” with a 4.2 magnitude earthquake and a fresh vent of steam.

On the morning of the now-historic eruption, a 5.2 magnitude earthquake triggered the collapse of a massive slab of the northern slope of the mountain. An enormous debris avalanche—the largest in recorded history—roared over the landscape. With a gigantic hole ripped down the volcano’s side, superheated gases and rock fragments exploded laterally/horizontally instead of vertically, a phenomenon that had not been witnessed or recorded before in the modern world.

The blast raged at hurricane speeds, reaching 200 to 250 miles per hour (320 to 400 kilometers per hour) at temperatures of 680°F (360°C), flattening and scorching trees. For more than nine hours after the blast, Mount St. Helens gushed an ash plume that reached 15 miles into the atmosphere. In 15 days, the plume circled the globe. Deadly pyroclastic flows, at least 1,300°F (704°C), flowed at a high speed from the crater and covered 6 square miles (15 square kilometers) under many feet of choking pumice.

The summit and northern slope were obliterated. The surrounding forest ecosystem was wiped out—trees burned, flattened or simply gone. Churning mudflows poured down the Toutle River, which was filled with trees, mud, boulders, and houses. When the roiling flow reached the Columbia River, it blocked the shipping channel. Fifty-seven people died, including David Johnston, the U.S. Geological Survey geologist who had been monitoring the volcano from the ridge that now bears his name.

After the eruption, Mount St. Helens delivered another surprise. Scientists expected life to return in predictable ways, following well-known stages in ecological succession, and thought that in some areas, nothing living could have survived. But that’s not how things happened. The timing and season of the eruption meant some nocturnal animals were protected in burrows, for example. Others had not yet arrived from migration. Dormant plants belowground also were protected. Rocky outcroppings, cliffs and snow patches sheltered some animals and plants.

Colonization and biological interaction in the blast area of the post-eruption landscape of Mount St. Helens created unique opportunities to learn how plants and animals respond to a massive, infrequent disturbance, which in turn has enabled scientists to understand and inform management plans for other disturbed areas. With its record of changes on the landscape year after year, scientists have used Landsat images to accurately characterize the pattern of ecosystem recovery after natural disasters.

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